Investment mixer



May 10, 1966 w. J. VAN ROSSEM INVESTMENT MIXER Filed March 16, 1964 IIIII'IIIIVI IIJ I NVEN TOR.

United States Patent 3,250,518 INVESTMENT MIXER Walter J. Van Rossem,1917 Thayer Ave, Los Angeles, Calif. Filed Mar. 16, 1964, Ser. No.352,068 5 Claims. (Cl. 259107) The present invention relates generallyto mixers for the plaster or investment material for investing waxpatterns; and the present invention is more particularly concerned withthe design of an improved bearing for the rotating shaft entering themixer and carrying the mixing arm inside the mixing bowl.

Apparatus of the type with which we are here concerned is used forinvesting a wax pattern, under vacuum, with a suitable composition whichwill harden and form a mold into which molten metal is poured to make acasting, for example in dental work, the manufacture of jewelry, andother fine arts. In this process, the materials to be mixed are a drypowder and water. The water and powder are mixed under vacuum to form aplastic mix which is then poured into the flask around the pattern. Theflask with the mixture in it is normally then vibrated on a vibratingtable.

These operations are normally carried out under a very high degree ofvacuum which is applied in order to eliminate any air bubbles that areliable to incur in the investment material and which, by adhering to theouter surface of the wax pattern, result in a rough or imperfectcasting. These bubbles are largely air which was originally dissolved inthe water added to the investment powder or mechanically included beforeor during mixing. By subjecting the investment mix to a high degree ofvacuum these occluded and dissolved gases are expanded and boiled out ofthe mix at a low temperature below the melting point of the wax pattern.Vibrating the investment material, before hardening, on a vibratingtable is a further aid to the elimination of air and vapor bubbles andto obtain an accurate pattern by causing the investment material toconform faithfully to the outlines of the wax pattern.

One of the problems previously encountered in mixers for investmentmaterial is maintenance of the bearing for the shaft which passesthrough the wall of the mixing bowl and carries inside the bowl the armwith which the investment material is stirred and mixed. The plaster mixis very highly abrasive and rapidly wears relatively moving parts. Whenthe vacuum is released at the end of the mixing operation, the pressuredifferentials may be in a direction to drive this abrasive mixture intothe shaft bearing. While this problem can be alleviated to a largeextent by cleaning the mixer thoroughly after each use, it will berealized that this discipline may not be observed in a commerciallaboratory where the mixer is in more or less continuous use. As aconsequence, the abrasive material is frequently allowed to get into thebearings. Once wear starts, clearances are increased and access of theabrasive mixture is easier,.thus increasing wear at a higher rate.

This condition creates a vicious cycle which not only wears the hearingbut eventually makes it impossible to maintain within the mixing bowlthe high degree of vacuum required because air can leak around therotating shaft. When this happens highly accurate work of the investmentprocess is no longer possible.

In some designs of mixer, the problem has been largely avoided by atop-mounting of the shaft bearing. That is, the shaft passes downwardlythrough the top wall of the mixing bowl and carries the stirring arm atits lower end, a design which is common in various mixing devices forpaint, salad dressing, and other familiar uses. However, in these casesthere is no problem of inverting the mixing Patented May 10, 1966 bowlin order to discharge by gravity the fluent contents of the bowl intothe molding flask.

In a mixer for investment material it is greatly preferred to have theshaft extend upwardly through the bottom wall into the bowl, thusrequiring the bearing to' be bottom-mounted. The rotating shaft is thenconnected at its lower external end to the drive shaft of a motor orother means for rotating the shaft and the stirring arm. Knownstructures of this type have suffered from the fact that the thrustbearing surfaces on the shaft are brought together by the downwardpressure exerted in holding the mixing bowl on the motor drive shaft,with the result that this thrust accelerates wear on the thrust bearingsurfaces, especially when investment material reaches these surfaces.

Thus it becomes the general object of the present invention to provide amixer suitable for investment material in which the shaft bearing ismounted at the bottom of the mixing bowl but is so constructed thatlittle or no investment material reaches the relatively movable metalsurfaces and consequently these'surfaces have a long life.

More particularly it is an object of the present invention to provide amixer for investment material in which the shaft bearings are adequatelysealed against entry of abrasive material so that the bearing, eventhough submerged in the investment material, is not accessible to theinvestment material.

A further object of the present invention is to provide in an investmentmixer a bearing construction of novel design in which the thrust bearingsurfaces exposed to the investment material are not pressed together,but rather tend to be separated by the normal thrust exerted on therotating shaft as it is held upon the drive shaft of the motor or othermeans rotating the shaft in the mixer.

These objects of the present invention are achieved in a mixer forinvestment material by providing a bowl, which is adapted to hold theinvestment material to be mixed, with a fixed sleeve projecting throughthe bowl wall, and a shaft rotatably mounted in the sleeve andprojecting beyond the sleeve at each end thereof. A stirring arm iscarried on the shaft at the end inside the bowl while the end of theshaft outside the bowl is adapted for connection to suitable means forrotating the shaft. Fluid sealing means between the shaft and the sleevenear the outer end of the sleeve provides a seal against entrance of airinto the bowl, thus permitting a high degree of vacuum to be maintained.in the bowl and a lubricant retaining device is placed inwardly of thesealing means from the shaft, the lubricant providing a seal againstaccess of investment material to the fluid sealing means. Outside thebowl, the sleeve-and shaft are provided with cooperating means, in theform of a pair of engaging thrust surfaces, which resist displacement ofthe shaft inwardly of the bowl by forces directed axially thereof. Apair of mutually opposing surfaces on the shaft and sleeve are locatedinside the bowl; but at this position the members of the latter pair ofsurfaces tend to be separated by the thrust on the shaft so thatinvestment material between these surfaces produces a minimum of wear.With this design, the shaft bearing can be mounted at the bottom of thebowl, allowing the outward end of the shaft to be below the bowl.

How the above objects and advantages of the invention, as well as othersnot specifically referred to herein, are attained will be betterunderstood by reference to the following description and to thedrawings, in which:

FIG. 1 is a longitudinal median view through an investment mixing deviceembodying the present invention.

FIG. 2 is a fragmentary view similar to FIG. 1 showing a portion of theinvestment mixing device in the inverted position occupied while theflask is being vibrated.

FIG. 3 is an elevation of the mixer shaft, alone.

Referring now to FIG. 1 of the drawing, the mixer illustrated thereincomprises a bowl 110 which is adapted to hold a quantity of investmentmaterial while it is being mixed with water. Bowl is normally open atone side or end as shown, the open side of the bowl being closed byhousing 12 within which flask 14 is mounted and by gasket 15 whichcloses one end of housing 12 and supports therein flask 14. Gasket 15also holds in place at one end of the flask the combined base and sprueformer 16. Gasket 17 between the bowl and the housing and gasket 15provide airtight seals between the separable parts of the mixer.

It will be noted that housing 12 provides an internal funnel structure18 which is designed to direct the investment mix into the upper end ofthe flask when the mixer is in the inverted or pouring position (FIG. 2)in contrast with the upright mixing position shown in FIG, 1. A suitablefitting 20 is provided on one wall of housing 12 for connection to avacuum line, not shown, in order that a suitable degree of vacuum can beapplied to the interior of the mixing device, including bowl 10.

Sleeve 22 passes through the wall of the bowl 10 and is preferablylocated at such a position that the sleeve passes through a portion ofthe bottom wall of the bowl when the mixer is in the upright mixingposition illustrated. Sleeve 22 is stationary and may be held in placeon the bowl wall by any suitable means, as by cement or the like, thatalso provides an airtight joint between the bowl and sleeve. Sleeve 22is metal in order to provide the desired strength and accuracy ofdimensions.

Inside sleeve 22 is shaft 23 which is freely rotatable within the sleeveand. which projects beyond the sleeve at both ends. At the lower orouter end of shaft 23 there is carried coupling 24 which is connected tothe shaft in any suitable manner, as by screw-threaded joint 25 so thatthe coupling may, for practical purposes, be regarded as a part of theshaft. The lower end of coupling 24 is provided with a transverse slot24a, or is otherwise suitably formed, in order to engage tongue 26 onthe upper end of the vertically extending shaft 27 which is driven by amotor, not shown in the drawing, either directly or through a suitablespeed-reducer. Shaft 27 serves as means for rotating shaft 23 of themixer.

Shaft 23 is provided with a shoulder 28 engaged by coupling 24 to limitthe travel of the coupling where it is screwed onto the shaft. Thedistance between shoulders 28 and 36, mentioned later, is such that thecoupling does not tighten and bend the shaft in sleeve 22.

At its upper end inside bowl 10, shaft 23 is provided with stirring armmeans 29. The stirring arm means may have any configuration desired andnormally consists of one or more lengths of wire bent into such a shapethat the wires thoroughly agitate and mix the investment material as thearm means passes through the material upon rotation of shaft 23.

First fluid sealing means between the shaft and sleeve are provided inthe form of O-ring 30 located near the outer or lower end of sleeve 22.Above and inwardly along shaft 23 from O-ring 30, the shaft is providedwith lubricant retaining means. This lubricant retaining means takes theform of a shallow, axially elongated recess cut in the peripheralsurface of shaft 23, as indicated at 31. For this purpose a recess depthof .005 inch has been found to be quite satisfactory as a recess of thisdepth holds an adequate amount of a reasonably stiff grease in a filmthat is strong enough to resist the pressure differential across it whenthe interior of the bowl is under a high degree of vacuum.

Above and inwardly of recess 31, there is preferably located a secondfluid sealing means in the form of O-ring 33 which is located near theupper end of sleeve 22. This second O-ring is preferably added to thebearing construction, although it is not necessary, in order to providea seal between the body of lubricant in groove 31 and the interior ofthe bowl. Thus the second fluid sealing means keeps the pressuredifferentials throughout the bearing down to sufiiciently low valuesthat there is no tendency for the lubricant to be pulled out of recess31 into the interior of the bowl and become mixed with the investmentmaterial.

It has been found experimentally that the first sealing means in theform of O-ring 30 located near the outer end' of the bearing sleevecombined with the lubricant film inwardly of the sealing means, arehighly effective for two different purposes. In the first place, thesetwo means prevent the entrance of outside air into the bowl as thevacuum is applied to bowl interior. Also, the body of lubricant ishighly effective 'in preventing investment material that gets into thebearing from passing through the film of lubricant and reaching thefluid sealing means at 31 Thus the lower O-ring is efliciently protectedby the lubricant against contamination by the investment material andthe clearances below or outwardly of the body of lubricant can bemaintained at all times sufiiciently small that the bearing is welllubricated and operates free of any abrasive material.

It will be noticed that the sleeve and the shaft rotating thereincooperatae to provide a pair of engaging thrust surfaces at 35. One ofthese surfaces is the lower end surface of the sleeve While the other isthe opposing end face on coupling 24 which, as mentioned above, may beconsidered as a portion of the shaft since it rotates with the shaft.conform in shape to each other and preferably lie in a substantiallyradial plane. It will. be noted that the thrust surfaces are locatedoutside of the bowl below sealing means 30 where the thrust surfaces maybe lubricated and are entirely free from contamination by the investmentmaterial. When the mixer is pressed downward on driveshaft 27 tomaintain a driving connection between shaft 27 and coupling 24, thedownward thrust on the bowl and the upward and inward reaction on shaft23 brings these thrust surfaces together. 1

Cooperating means on the shaft and sleeve provide two opposed surfacesat the inner end of the sleeve as indicated at 36; but it will be seenthat these same thrust forces just mentioned tend to separate the twosurfaces at 36 rather than to press them together. At the upper end ofthe sleeve, the end face of the sleeve is opposed by a downwardly facingshoulder formed on the shaft near its upper end, as shown in FIG. 3,these two surfaces preferably being in a radial plane; The axial lengthof sleeve 22 is such that the two surfaces at 36 are spaced well abovethe bottom of the bowl and are near the average level of the surface ofthe investment material during the mixing operation.

Actually there is no fixed position for the level of the surface of theinvestment material since if only a small amount is being mixed, the mixlevel is normally down below the surfaces at 36, whereas if a largequantity of investment material is required and is being mixed, thelevel of the mix will be somewhat above surfaces 36.

- However, on the average the amount of investment material being mixedin the bowl at any time brings the surface level approximately at thelocation of opposed surfaces 36.

This location of these opposedsurfaces which between them provide accessto the bearing interior, has several advantages. In the first place, anyair which may work out of the bearing into the investment materialpasses through only a small portion of the. investment material, if atall. Also there is very little tendency for any of the investmentmaterial to be forced into the bearing when the joint 36 is near the topof the body of investment material. In any event any investment materialworking into this joint between the surfaces on the end of the sleeveand on the bearing has a minimum abrasive effect because the axialforces on the sleeve are in a direction such that these surfaces tend toseparate rather than to approach one another with the result that theabrasive Thesetwo engaging surfaces indicated at 35 compound does notabrade the opposing surfaces to any significant extent.

Also the level of the surface of the investment material at the centerof the bowl depends upon the speed with which the stirring arm means isrotated. A speed in the vicinity of 400 rpm. may be considered astypical or average; but sometimes drive shaft 27 is rotated at speeds ashigh as four to five times this figure, as a speed of 1725 r.p.m. is nowcommonly used. At this considerably higher speed, the investmentmaterial is thrown outwardly against the walls of the bowl and there isa corresponding lowering of the level of the material at the center ofthe bowl.

After the investment material is thoroughly mixed, the mixer isdisconnected from driveshaft 27 by lifting it off the driveshaft and theentire mixer is inverted, allowing the mixed investment material to flowby gravity through funnel 18 into flask 14 and around the pattern 40mounted in the bottom of the flask, all as is well known in the art.Although it will be understood that the mixer may be in this position atthe time the vacuum is broken, the investment material adhering to theshaft and sleeve end portions thereof is in a position to he forced intothe bearing at the time that normal pressure is restored inside thebowl, if the design of the bearing structure permits.

It is generally recognized in the investment casting art that the mixeris the weakest link in the chain of equipment required for this process.The bearing for the rotating shaft is, as has been already pointed out,the sensitive point of this piece of equipment; and it has beenparticularly characteristic of submerged or bottom-mounted bearingstructures that they wear out rapidly. This results in unnecessaryexpense to the user or poor technique, or in some cases both. On thecontrary, the present design has been tested and developed undercontinuous laboratory operation and conditions similar to thoseencountered in commercial laboratories and has been found to possess anunexpectedly long life. This is attributed to the continuous lubricationof the bearing and the lack of any abrasive material between surfacesthat are caused to approach and bear against one another during themixing operation. As a result the bearing tolerance has remained closeand the fluid tight seals remained efiicient and able to hold a highdegree of vacuum. All this contributes not only to the longevity of thispiece of equipment but .to the improved accuracy and satisfaction ofoperation of the entire technique of the investment casting.

It will be realized that various changes may be made in the details ofdesign and arrangement of the elements of the present invention bypersons skilled in the art without departing from the spirit and scopeof the present invention. Accordingly, it is to be understood that theforegoing description is considered as being illustrative of, ratherthan'limitative upon, the invention as defined by the appended claims.

I claim:

1. Vacuum investing apparatus, comprising:

a bowl adapted to hold a quantity of investment material to be mixed andopen at one end;

closure means for said one open end including investment discharge meansand means for connecting to a vacuum line to maintain the interior ofthe apparatus under a partial vacuum;

a flash positioned by engagement with the closure means to receiveinvestment material from said discharge means when the apparatus isinverted;

a fixed incompressible sleeve passing through the bowl wall at aposition located at the bottom of the bowl when mixing is carried on,said sleeve extending upwardly into the bowl to a location near theaverage surface level of investment material in the bowl during mixing;

a generally upright rigid shaft rotatably mounted in the sleeve andprojecting beyond the sleeve at each end thereof, said shaft beingadapted at its lower end for connection to means for rotating the shaft;

stirring arm means on the shaft at the upper end thereof;

said sleeve and said shaft having cooperating means providing a pair ofengaging thrust bearing surfaces located at the lower end of the sleeveoutside the bowl, said surfaces resisting displacement of the shaftupwardly relative to the bowl by forces directed axially thereof andinwardly of the bowl;

and fluid sealing means between the shaft and the sleeve.

2. A mixer as in claim 1 in which the fluid sealing means comprises apair of separate sealing elements spaced along the shaft and a shallowannular recess between and spaced from each of said sealing elementsproviding a lubricant retaining means between the sealing elements.

3. A mixer as in claim 1 in which the shaft has an outwardly extendingsurface opposing an end surface of the sleeve, said two mutuallyopposing surfaces being so arranged that they tend to separate under theinfluence of a downward axial force on the shaft maintaining engagementof the shaft with means for rotating the shaft.

4. Vacuum investing apparatus, comprising:

a bowl adapted to hold a quantity of investment material to be mixed andopen at one end;

removable closure means for said one open end including investmentdischarge means and means for connecting to a vacuum line to maintainthe interior of the apparatus under a partialvacuum;

a flask positioned by engagement with the closure means to receiveinvestment material from said discharge means when the apparatus isinverted;

a continuous incompressible sleeve passing through the bowl wall at aposition located adjacent the bottom of the bowl when mixing is carriedon, said sleeve extending upwardly into the bowl through investmentmaterial in the bowl during mixing;

a generally upright rigid shaft rotatably mounted in the sleeve andprojecting beyond the sleeve at each end thereof, said shaft beingadapted at its lower end for connection by relative axial movement tomean for rotating the shaft;

stirring arm means on the shaft;

fluid sealing means between the shaft and the sleeve;

said sleeve and shaft having means providing a first pair of opposedsurfaces adjacent the upper end of the shaft separated by relativeupward movement of the shaft;

and said sleeve and shaft also having means providing a second pair ofengaging thrust bearing surfaces located at the lower end of the sleeveoutside the bowl, said surfaces resisting displacement of the shaftupwardly relative to the bowl and separation of the first pair ofsurfaces by forces directed axially of the shaft and inwardly of thebowl.

5. A mixer for investment material as in claim 4 in I which the firstpair of opposed surfaces are located near or above the average surfacelevel of investment material in the bowl during mixing.

References Cited by the Examiner UNITED STATES PATENTS 1,480,914 1/ 1924PoplaWski 259-108 2,660,210 11/ 1953 Berglind 259-107 X 2,696,022 12/1954 Steinbock et al 22-35 X 2,777,177 1/ 1957 Steinbock et a1 22-352,794,627 6/ 1957 Rodwick 259-108 2,930,596 3/ 1960 Waters 259-108 X2,992,715 7/ 1961 Blachly 146-68.1 X

CHARLES A. WILLMUTH, Primary Examiner.

E. L. ROBERTS, Assistant Examiner.

1. VACUUM INVESTING APPARTUS, COMPIRISING: A BOWL ADAPTED TO HOLD AQUANTITY OF INVESTMENT MATERIAL TO BE MIXED AND OPEN AT ONE END; CLOSUREMEANS FOR SAID ONE OPEN END INCLUDING INVESTMENT DISCHARGE MEANS ANDMEANS FOR CONNECTING TO A VACUUM LINE TO MAINTAIN THE INTERIOR OF THEAPPARATUS UNDER A PARTIAL VACUUM; A FLASH POSITONED BY ENGAGEMENT WITHTHE CLOSURE MEANS TO RECEIVE INVESTMENT MATERIAL FROM SAID DISCHARGEMEANS WHEN THE APPARATUS IS INVERTED; A FIXED INCOMPRESSIBLE SLEEVEPASSING THROUGH THE BOWL WALL AT A POSITION LOCATED AT THEBOTTOM OF THEBOWL WHEN MIXING IS CARRIED ON, SAID SLEEVE EXTENDING UPWARDLY INTO THEBOWL TO A LOCATION NEAR THE AVERAGE SURFACE LEVEL OF INVESTMENT MATERIALIN THE BOWL DURING MIXING; A GENERALLY UPRIGHT RIGID SHAFT ROTATABLYMOUNTED IN THE SLEEVE AND PROJECTING BEYOND THE SLEEVE AT EACH ENDTHEREOF, SAID SHAFT BEING ADAPTED AT ITS LOWER END FOR CONNECTION TOMEANS FOR ROTATING THE SHAFT; STIRRING ARM MEANS ON THE SHAFT AT THEUPPER END THEREOF; SAID SLEEVE AND SAID SHAFT HAVING COOPERATING MEANSPROVIDING A PAIR OF ENGAGING THRUST BEARING SURFACES LOCATED AT THELOWER END OF THE SLEEVE OUTSIDE THE BOWL, SAID SURFACES RESISTINGDISPLACEMENT OF THE SHAFT UPWARDLY RELATIVE TO THE BOWL BY FORCESDIRECTED AXIALLY THEREOF AND INWARDLY OF THE BOWL; AND FLUID SEALINGMEANS BETWEEN THE SHAFT AND THE SLEEVE.